High Intensity Discharge (HID) lamps are high-efficiency lamps that can generate large amounts of light from a relatively small source. These lamps are widely used in many applications, including highway and road lighting, lighting of large venues such as sports stadiums, floodlighting of buildings, shops, industrial buildings, automotive headlamps and video projectors, to name but a few. The term “HID lamp” is used to denote different kinds of lamps. These include Mercury Vapor lamps, Metal Halide lamps, and High Pressure Sodium lamps. Metal Halide lamps, in particular, are widely used in areas that require a high level of brightness and excellent color quality at relatively low cost. HID lamps differ from other types of lamps because their functioning requires operation at high temperature and high pressure over a prolonged period of time. Also, due to their usage and cost, it is desirable that these HID lamps have relatively long useful lives and produce a consistent level of brightness and color of light. Although in principle, HID lamps can operate with either an alternating current (AC) supply or a direct-current (DC) supply, in practice, the lamps are usually driven via an AC supply.
Discharge lamps produce light by ionizing a mixture of gaseous and vapor phase fill material, such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel, which defines an interior chamber also called as a discharge chamber. The sealed discharge chamber maintains the pressure of the energized fill material and allows the emitted light to pass through its translucent or transparent wall. The fill material, also known as a “dose,” emits a desired spectral power density distribution in response to being excited by the electric arc. For example, metal halides provide spectral power density distributions that offer a broad choice of light properties, e.g. color temperatures, color rendering indices, and luminous efficacies.
Such lamps often have a high initial light output that diminishes considerably over time basically due to blackening of the discharge chamber walls. The blackening is principally caused by tungsten and tungsten alloy particles of the electrode material transported from the electrodes to the discharge chamber wall. It has been proposed to incorporate a calcium oxide or tungsten oxide oxygen dispenser in the discharge vessel, as disclosed, for example in WO 99/53522 and WO 99/53523 by Koninklijke Philips Electronics N.V. Lamps produced according to the proposals in these applications may not, however, simultaneously meet increased expectations set against lamp efficacy, color hue and color quality of emitted light, color consistency and temporal color stability, luminous flux maintenance over useful lamp life, and reliability measures for a commercial lamp.
In addition to the issues associated with discharge chamber wall blackening, improved startability (i.e., reduced starting time, increased starting reliability, hot re-start capability, etc.) of HID lamps has recently become an important problem in the art. In this regard, lamp constructions having both good startability and high performance under steady-state operating conditions have required some compromise. This is largely due to the fact that physical, chemical and electrical conditions of the lamp at these two different phases of operation are considerably different.
Initially, the gas fill contained in the discharge vessel of a discharge lamp is electrically non-conductive. If an electric potential is applied on the electrodes of the lamp, this creates a favorable situation to strip the outer orbital electrons from the atoms of the gas fill (ionization of gas atoms) or from the crystal lattice of the electrode material and thus create free electrons, which are then accelerated though the gas by the electric field generated between the electrodes. This initiates the creation of more free electrons by collision with other gas atoms, which in turn are also ionized. If the applied electric field strength is high enough, high fraction of new electrons thus created will create additional electrons by inelastic collisions with gas atoms and ions in the fill, and initiate an electron avalanche. Such an avalanche finally creates the self-sustaining electric discharge in the lamp. However, to create such free electrons by simple dielectric breakdown of the gas fill by the strong electric field requires several tens of kilovolts of electric potential to be applied to the electrodes. Higher electric potentials require more expensive external electrical circuitry, and may not be commercially feasible. Unwanted breakdown can also occur in the outer jacket and in the cap-base region of the lamp, which may even completely inhibit starting.
Discharges for commercial lighting applications employ an additional initial source of free electrons, which removes the need for generating such high voltages to initiate the phase of discharge formation. Such external sources can be a heated filament, use of ever present cosmic rays, or providing a source of electrons by radioactive decay. Heated filaments are not practical in High Intensity Discharge (HID) lamps, and the cosmic ray background radiation is usually insufficient or of unreliably random nature to dramatically reduce the need for very high electric fields needed to initiate lamp ignition, unless other methods are used to lower the breakdown voltage.
For providing an initial source of free electrons by radioactive decay, typically what has been used in the past in the HID discharge vessel is a radioactive gas, such as Kr85 with most of the decay products being beta particles (i.e., electrons). Kr85 has a half-life of 10.8 years, with 99.6% of the decay products being beta particles (i.e., electrons) having a maximum kinetic energy of 687 keV. These electrons have very high energy, and in many respects are ideal sources for free electrons and are used widely as such for these applications. But to provide enough of these high energy electrons by radioactive decay, a significant quantity of this gas has been used in HID lamps.
The presence of Kr85 in such lamps diminishes the need for providing very high electric potential on the electrodes, which makes the external electrical circuitry (i.e., a ballast) and systems design simpler and the whole lighting system more cost effective. Typical applications use such a radioactive gas with a starter/ignitor unit built into or applied separately along with a ballast that provides a high electric pulse for a very short duration of time, typically in the millisecond (or several hundred microseconds) range, which is very effective in creating the electron avalanche referred to earlier. However, recent UN2911 government regulations limit the amount of radioactive Kr85 used in lamps. These regulations proscribe the HID lamp manufacturers from using the large quantity of Kr85 gas that has been previously used, as described in preceding paragraph. Consequently, the minimization and/or elimination of Kr85 from the fill gas of HID lamps is now required.
This disclosure provides a new and improved Metal Halide lamp with improved luminous flux maintenance over useful lamp life, startability and performance under steady-state lamp operation.